Nico Rosberg was fastest in both practices for the Brazilian Grand Prix.

AFP: Andrew Yates, file photo

Nico Rosberg made the most of a limited supply of tyres on Friday when he topped the times for Mercedes in both free practice sessions for Monday's (AEDT) Brazilian Grand Prix.

The 28-year-old German, who had led a Mercedes one-two ahead of team-mate Lewis Hamilton in the morning, was fastest again, outpacing both Red Bulls in the final minutes of another rain-hit session in the afternoon.

For Rosberg it was confirmation of his merit among the top drivers, many of whom were left frustrated by the lack of available suitable Pirelli tyres in the conditions.

"It would have been nice to go out and do more laps but we don't have the tyres. It's not good for the fans," explained 2008 champion Hamilton.

The Briton added he was unable to match Rosberg's time because he did not feel as comfortable or competitive in the wet in the Mercedes car as he wished.

"I have been looking back on my career and I don't think I have ever felt so uncomfortable in the wet in any car," said Hamilton.

"But Nico has shown the speed is there so I just have to work out what it is for me."

As a result of the conditions and the meagre tyre supply most teams only ran with one set of tyres in order to preserve more for Sunday's practice and the decisive qualifying session for the season-ending contest at Interlagos.

"It's just the way it is," said four-time world champion German Vettel, who was second for Red Bull.

"It's not good for the people out there in the grandstands, but there's nothing we can do."

Mark Webber, preparing for his last race in Formula One, was third for Red Bull and said he had done his best to prepare for what lies ahead in challenging conditions on a day of virtually incessant rain.

Rosberg, who clocked one minute and 24.781 seconds in the morning, was on top for most of the afternoon in wetter conditions until it dried sufficiently for the top drivers to risk using intermediate tyres.

This brief late period saw Vettel setting the pace ahead of Webber.

But Rosberg bounced back to top the times again in 1:27.306, two-tenths of a second faster than Vettel.

The champion was second ahead of Webber with Finn Heikki Kovalainen fourth for Lotus, where he has replaced injured compatriot Kimi Raikkonen for the final two events, and Hamilton fifth.

Frenchman Jean-Eric Vergne was an impressive sixth for Toro Rosso ahead of local hero Brazilian Felipe Massa, in his last race weekend with Ferrari, German Nico Hulkenberg of Sauber, and Australian Daniel Ricciardo, in his last outing with Toro Rosso before moving to Red Bull next season.

Frenchman Romain Grosjean was 10th in the second Lotus ahead of two-time world champion Spaniard Fernando Alonso in the second Ferrari.

Jenson Button made no impression for McLaren, who appeared to be prepared for dry conditions rather than wet, and he wound up down in 22nd place, five behind his team-mate Sergio Perez.